1. Field of the Invention
The present invention relates to semiconductor switching devices and more particularly to two terminal thyristor devices or reverse switching rectifier devices, hereinafter designated RSR devices.
2. Description of the Prior Art
RSR devices of the prior art have a general structural configuration shown in FIG. 1 wherein a body of semiconductor material in the form of a wafer 10 is doped to provide four alternate semiconductivity zones. An end zone 12 of P-type semiconductivity extends from one major surface 11 of the wafer 10 into the semiconductor material to meet a middle zone 14 of N-type semiconductivity. PN junction 13 is formed at the interface of zones 12 and 14. Similarly, P-type middle zone 16 forms PN junction 15 with zone 14. Zone 16 extends from PN junction 15 to end zone 18 of N-type semiconductivity located in an inner portion of the wafer 10 where PN junction 17 is formed. In addition, zone 16 typically extends past zone 18 to the outer portion of major surface 21. Zone 16 serves as the base of the device, and zone 18 serves as the emitter. Typically, a shorted emitter construction is used whereby a cathode electrode 22 is affixed to major surface 21 contacting the emitter zone 18 and a peripheral portion of the base zone 16 surrounding the emitter zone 18. The electrode 22 may be provided, for example, by aluminum deposition in a known manner. A supporting anode electrode 24 is affixed to major surface 11 to provide good electrical and thermal contact to zone 12 as well as to provide mechanical support for the wafer 10. Typical examples of metals used for the electrode 24 are molybdenum and tungsten, which are preferred for their favorable expansion properties. The wafer 10 has a beveled edge 25 produced in a known manner in order to optimize electrical characteristics. Disposed on the beveled edge 25 is an insulating and protective coating 26. The coating composition and manner of application is known in the art, a high temperature curing silicone varnish being an example of a suitable coating material.
The RSR device of the prior art shown in FIG. 1 operates as an electrical current switch. Briefly described, the RSR blocks voltage in both directions unless the device is turned on in which case it carries current in the forward direction as indicated by the arrow 27. The RSR device may be turned on in the presence of a forward voltage, as indicated by the polarity marks + and -, by impressing a forward voltage pulse across electrodes 24 and 22, which pulse has a sufficiently high DV/DT to cause the device to turn on.
It has been found that device structures of the prior art, as shown in FIG. 1, do not turn on uniformly along PN junction 15, rather such prior art devices initially turn on in a relatively small region located under an edge of the emitter zone 18 causing hot spotting and failure of the device. An example of a typical failure mode is illustrated in FIG. 1 in which emission of electrons from zone 18 into zone 16 more readily occurs in the dashed region 28 causing initial conduction of current through PN junction 15 to pass through the relatively small area of region 28 as illustrated by path 29. The very high current density along path 29 causes localized heating which permanently destroys the blocking capability of PN junction 15.
The present invention provides an RSR device which turns on uniformly thus eliminating the above-mentioned failure mode of prior art devices.